(1) Field of the Invention
The present invention relates to focus control devices and focus control methods and in particular to a focus control device and a focus control method wherein the driving time of a focus lens can be shortened to perform quick focusing operation.
(2) Description of the Related Art
Many image pickup devices including monitoring cameras (for prevention of crimes) and DVD (Digital Versatile Disc) cameras are equipped with an auto focus (hereafter, abbreviated as AF) function for automatically adjusting focus.
Various methods have been conventionally proposed and brought to practical use for auto focusing of image pickup devices. Among these methods, a contrast detection method in which a picture signal obtained through an image sensor is processed to detect a contrast signal in the picture signal is in wide use.
A high-frequency component of a picture signal existing in a contoured part or the like when an image of a subject is picked up is increased as it comes into focus. This is equivalent to increase in the difference in brightness level, that is, increase in contrast between a bright section and a dark section in the contoured part of the subject. The contrast detection method makes good use of this property and to bring a desired subject into focus, a focus lens only has to be controlled in a direction in which a high-frequency component of a picture signal is large.
This detection method is also designated as hill climbing method. This is because the following takes place when a focus lens is moved and a high-frequency component is extracted from a picture signal obtained at each lens position with HPF (High Pass Filter) to compute an AF evaluated value: such a chevron characteristic that the AF evaluated value in the proper focus position comes to the peak (the summit of a hill) is exhibited.
With HPF high in cutoff frequency, the stable peak value (a crest of AF evaluated value) of AF evaluated value can be obtained from a subject, such as a night scene, involving a point light source. In case of a subject low in contrast, the slope of the crest of AF evaluated value becomes gentle and it is difficult to obtain a proper point of focus (the lens position where the AF evaluated value comes to the peak). With HPF low in cutoff frequency, a crest of AF evaluated value can be obtained from a low-contrast subject. However, in case of a subject, such as a night scene, involving a point light source, a pseudo peak designated as pseudo crest is prone to occur and it is difficult to obtain a proper point of focus.
To cope with this, JP-A-2007-108412 discloses a technique. In this technique, the following processing is carried out when it is determined that both a high-brightness region and a low-brightness region exist in a focus area: a point light source is considered to exist and the setting of the cutoff frequency of HPF is changed from low to high to enhance focusing accuracy.
JP-A-2005-122016 discloses the following technique carried out with respect to such a subject as a night scene in which multiple point light sources are visible: an AF evaluated value is corrected according to the proportion of a high-brightness image occupying a picked-up field image and a focused focal point is identified based on the corrected AF evaluated value.